https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81541
Bug ID: 81541
Summary: Potential size optimisation: reusing common function
endings
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: other
Assignee: unassigned at gcc dot gnu.org
Reporter: jpakkane at gmail dot com
Target Milestone: ---
In embedded development code size optimizations are very important and there
are some optimizations that commercial compilers do but GCC does not seem to.
As an example compiling the following code:
int func5() {
int i = 0;
i+=func2();
i+=func3();
i+=func4();
return i+func1();
}
int func6() {
int i=1;
i+=func3();
i+=func3();
i+=func4();
return i+func1();
}
On ARM using -Os produces the following assembly:
func5():
push {r4, lr}
bl func2()
mov r4, r0
bl func3()
add r4, r4, r0
bl func4()
add r4, r4, r0
bl func1()
add r0, r4, r0
pop {r4, lr}
bx lr
func6():
push {r4, lr}
bl func3()
add r4, r0, #1
bl func3()
add r4, r4, r0
bl func4()
add r4, r4, r0
bl func1()
add r0, r4, r0
pop {r4, lr}
bx lr
Note how the ends of both of these functions have identical endings. This could
be converted to the following:
func5():
push {r4, lr}
bl func2()
mov r4, r0
common_tail:
bl func3()
add r4, r4, r0
bl func4()
add r4, r4, r0
bl func1()
add r0, r4, r0
pop {r4, lr}
bx lr
func6():
push {r4, lr}
bl func3()
add r4, r0, #1
b common_tail
which has smaller code size.
I did a simple experiment and based on that this might not be all that useful
on x86:
http://nibblestew.blogspot.com/2017/07/experiment-binary-size-reduction-by.html
On ARM and similar platforms this would probably be beneficial, especially
given that commercial compilers perform this optimization.
